Various methods for terrestrial transmission of digital broadcast signals are known, having modulation types like OFDM, QPSK and QAM. A main concern in connection with such systems is to find a system-conform signal in case a receiver is switched on or tuned to another channel. Examples for such broadcast signals are DVB (digital video broadcast), HDTV-T (hierarchical digital television transmission) and DAB (digital audio broadcast) signals.
Considerations on DAB receiver tuning concepts are so far based on the assumption that an acceptable number of fixed DAB center frequencies can be used, if necessary with small offsets of quite below the 16 kHz step size. A reasonable approach is given by the CEPT (conference Europeenne des administrations des postes et des telecommunications) T-DAB allotment plan of July 1995. This approach would result in a reasonable time to scan the entire set of DAB frequencies and would require an acceptable size of extra memory for storing the corresponding numbers or to provide an adequate algorithm for the calculation during the tuning process.
In addition to the CEPT plan, broadcasters and national organisations are considering a lot of exceptions or, in general, offsets of up to several 100 kHz. The main reason behind these proposals is to reduce the influence from and to other services in the same or neighbouring frequency sections. Such scenario would increase the amount of fixed channel numbers to be tuned and subsequently the size of the receiver memory and the scanning time for the entire frequency set.
An accompanying proposal made by the industry is--in order to minimise the constraint for the receiver design--that all transmitters are obliged to transmit all other DAB transmitter frequencies of a wide area around that one which provides the information. However, it is questionable whether such method is practicable.